Sadly, amplifying an AM signal isn't trivial. Your amplifier needs to be linear, or you'll get gross distortion and lots of interference. That design of MW transmitter board has been around for a while, and uses the usual three - IC synthesiser trick and a CMOS switch IC as the final stage. Its harmonic purity isn't too great - OK at the few milliwatts it generates, but going to get you into trouble if you amplify it.

There is a very simple way of getting a few Watts on Medium Wave for just a few quid. You need the three-chip PLL - a 4060, a 40103 and a 4046. Using a 4608 kHz crystal (Farnell sell them), you'll get 9 kHz steps up the band (correct for Europe). The synthesiser will give you a squarewave at the output frequency. You feed that into the gate of a power FET - something like an IRF510 (80p from Farnell). The source of the FET is grounded, and the drain goes through a simple transformer (on a ferrite toroid) to the modulated supply. The other transformer winding goes through a lowpass filter (also wound on ferrite rings) to the aerial. The simplest modulator is an NPN power transistor driven by an op-amp, but you can also use the power op-amp audio ICs (I like the TDA2030), which will give ½supply when quiescent. The most expensive single part of the whole rig will be the heatsink!

My little MW rigs are about the size of a shoebox and come in 10/50 Watt and 20/100 Watt versions (that's Carrier and PEP ratings). The smaller one runs off a single car battery, and the bigger one uses two car batteries (in series). If anyone wants to give it a go, I'll put the circuit diagrams up here, along with some simple suggested wire aerials.

Medium Wave has plenty of room now that the French have gone. We used to get daytime coverage of much of south-east England with just 20 Watts carrier into a quarter-wave sloper!

I got a great location nearby for medium wave, Can put a wire dipole up between two trees and it's over a river and has mains 247, good thing is nobody goes there as it's all fenced off.
Did try Shortwave from it once on lower power, but never could get hold of the correct modulation transformer to bring the audio and power levels up.

This one uses a 40103, 4046, 4053, 4060, LM358, and a 9.216 Mhz Xtal, and it covers all MW with 9Khz steps.

If I were to make the antenna a 1/4 wave in length though just as an experimental test, do you think a fair bit more than 150 metres range could be accomplished, or do you reckon there'd be very little difference compared to the time I used a 7 ft length of wire?

Why don't you build a simple MW rig? I'll post circuits and layouts on here if you're interested. You can get a simple 2 Watts carrier / 10 Watts peak together for about £15 (if you don't buy from Maplin!). If you put that into a ¼-wave Marconi aerial with a good earth, you'll go a few miles in the daytime!

I've been supplying little car battery powered MW rigs to Africa for a while. They're supplied with a built-in USB input MP3 player (£4.50 from China) and have basic audio processing. If you don't plug anything into the mod socket, it plays from a USB stick. The only adjustments are internal - frequency setting using an 8-way DIP switch and "match" which adjusts the coupling to the antenna. These come in three power outputs. The most popular is the 10 / 50 Watt, but the 4 / 20 Watt is increasing in popularity.

Ive built mine, its ok for a bit of fun. As I understand it the small inductors on the circuit board (used to match the short antenna) are quite lossy by nature. I will have a go winding something more efficient on a ferrite rod and see what difference it makes - purely for fun of course

I know it's a little late, as the initial post on was over a year ago. Maybe it's so obvious that no one has pointed it out, but the circuitry Albert is describing is half built....

You can use your small AM rig as the PLL oscillator (it is already the 4046, 40103 and 4060 circuitry that Albert describes) and add to it an amplifier and modulator as described by Albert, just remove the analogue switch IC and take the carrier to your (newly built) amplifier and modulator.

Below is circuitry that Albert always seems to write about - (From Biascomms) - freely available online. You may wish to upgrade some of the transistors to more modern parts, instead of hunting for the old and possibly hard to find.

You can search 'medium wave alliance' in Google to find the details if you don't mind being bombarded with junk advertising and possibly worse.

Amplifier and modulator:

xmitter1c_pa&amp;mod.gif

Audio processing:

xmitter1b_audio.gif

I do wonder about the interference potential of AM rigs to poorly screened (plastic cased) audio equipment though, which might end up being a problem.

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The big trick is to find a site away from houses! This can be difficult in a city, but there's a lot of "Common Land" just outside most cities and towns.

One site I used at the top of the medium wave band (1593kHz) was a very tall, brick built water tower. I installed a "sloper", tied off to the top of a nearby lamp post, and used the water pipes to the tank at the top as the earth / counterpoise. The rig was housed in a cupboard in the pump room beneath the tank, and powered from the mains supply that drove the pumps and room lighting. The link receiver was a satcan and just an LNB, and the source site was just a couple of hundred metres away in a barn. The programmes all came off tape (later replaced with a computer), and the coverage - even at night - was fabulous. I ran just under 100 Watts PEP with tightly compressed mod. It sounded pretty good too. The set-up lasted for over five years before the authorities decided that they wanted to take it off.

So finally made an antenna for the transmitter. Built a Inverted L with Loading coil.

Fed with RG-8mini to a earth rod and one radial about 20M long, the inner core goes to a wire which has a has vertical section 5.5M then across 1M to a loading coil after the loading is 12M of wire. Best SWR 1.3

I can only get a max 600mW measured on my SX-600 VSWR meter. Not really fit for purpose the meter!

But can hear it 1mile away on my poor car stereo. Every station on AM/FM is weak, think the front end has had it.

When I plug in the antenna to my comms RX not many clear frez during the night!

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There are a few clearer channels about - a look on a website like fmscan.org should help you pick a few to check with a good receiver. I like 1512 KHz for instance, where I am there is nothing on it but crackles and the very occasional burst of something arabic...

So finally made an antenna for the transmitter. Built a Inverted L with Loading coil.

Fed with RG-8mini to a earth rod and one radial about 20M long, the inner core goes to a wire which has a has vertical section 5.5M then across 1M to a loading coil after the loading is 12M of wire. Best SWR 1.3

I can only get a max 600mW measured on my SX-600 VSWR meter. Not really fit for purpose the meter!

But can hear it 1mile away on my poor car stereo. Every station on AM/FM is weak, think the front end has had it.

When I plug in the antenna to my comms RX not many clear frez during the night!

I made a post about output power in relation to frequency re' the Mosquito, but the post seems to have vanished along with lots of other posts from around March/April.

I no longer have the Mosquito, and I'm not sure what frequency you were doing your ^ test on, but I found the output power varied quite dramatically according to frequency on mine; If I remember correctly, I was seeing 500 > 600mW on the lower and upper part of the bands, and around 900mW between 1100 to around 1250 Khz, dipping off very quickly out of this range or thereabouts.

I made a post about output power in relation to frequency re' the Mosquito, but the post seems to have vanished along with lots of other posts from around March/April.

I no longer have the Mosquito, and I'm not sure what frequency you were doing your ^ test on, but I found the output power varied quite dramatically according to frequency on mine; If I remember correctly, I was seeing 500 > 600mW on the lower and upper part of the bands, and around 900mW between 1100 to around 1250 Khz, dipping off very quickly out of this range or thereabouts.

Have a check around 1200Khz and see what power output you get.

Thanks for the info. Currently it's on 1494khz as daytime I don't hear anything but evening time Lazer pops up so I switch off.
I did find at spot freqenices there was higher output but no clear freq round here to use . Still testing might try another antenna and another dial as I have a 18m roach pole and see if I can do a just do a vertical only lucky my antenna analyser goes down that far.